Structures and Properties of TiAl-Base Alloys Containing Mn
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STRUCTURES AND PROPERTIES OF TiAl-BASE ALLOYS CONTAINING Mn T. TSUJIMOTO* AND K. HASHIMOTO* *National Research Institute for Metals,
Nakameguro,
Tokyo,
Japan
ABSTRACT Structures and mechanical properties of TiAl-base alloys have been studied, in which Zr, Nb, V or Mn is added as the third element and the Al content is changed. Behavior of the third element in the TiAl-base alloys and effects of various factors on ductility have been discussed. It has been concluded that the Ti-48.4 at% Al-1.0 at% Mn alloy is the optimum composition for obtaining good room temperature ductility. INTRODUCTION In order to use the compound TiAl as a structural material at high temperatures, we need to improve ductility at room temperature first of all. To achieve this purpose, we studied structures and mechanical properties in TiAl-base alloys of various compositions containing Zr, Nb, V and Mn as the third element. Our interest was in microstructures and occupation sites of the third elements in TiAl for reasons described in the next paragraph. It was found that Mn is the only element which occupies mainly the Al site of TiAl, and that the addition of Mn and the decrease of the Al content in the alloys promotes deformation of TiAl by twin formation [1,2,3]. The best ductility at room temperature was obtained for a 2-phase (TiAl+Ti3Al) alloy containing Mn [4]. A large ingot of this alloy was made to verify the good ductility, which is described in the last section of this paper. This study is currently in progress [5]. As TiAl fractures by cleavage at room temperature, we anticipate that fine microstructures will have a beneficial effect on the room temperature ductility [6,7]. In this paper methods to make microstructures fine are described later. Al atoms are known to have a tendency to absorb electrons of other elements in certain circumstances. The character of TiAl is difficult to comprehend without taking this tendency of the Al atoms into consideration, that is, only in terms of the interaction parameter as applied to metallic alloys. This compound has the crystal structure of L1, with c/a>1, and moreover the cleavage does not always occur at (002) planes for which the atomic distance is large [8]. From these facts we have to consider that covalent-like bonding is formed between the Al atoms and the nearest Ti atoms, and that this covalent-like bonding decreases the number of conduction electrons in TiAl [9]. Based on the above consideration, it is also anticipated that decreasing the Al content in TiAl or replacing the Al atoms by more metallic elements would produce a good effect on room temperature ductility, because an increase of the available conduction electrons would produce a rise in the cleavage strength. EXPERIMENTAL PROCEDURE Button ingots of 100g were made using an argon arc melting furnace with a water-cooled copper crucible. For the study described in the last section ingots of 50kg and 6kg were used, Mat. Res. Soc. Symp. Proc. Vol. 133. c1989 Materials Research Society
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which were melted by a vacuum arc
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